1. Field of the Invention
The present application relates to an injection mold that minimizes a residual stress of a molded light guide plate, a light guide plate fabricated using the same, and a Liquid Crystal Device (LCD) device having the light guide plate.
2. Description of the Related Art
An LCD is advantageous over a Cathode Ray Tube (CRT), because an LCD can be made to have a small size, is light-weight and provides a better overall picture quality. Thus, LCDs are now used in many devices such as televisions, computers, etc. Further, because the LCD is a non-luminous type device, the LCD includes a backlight unit that applies a light to the rear of the LCD panel.
In addition, the backlight unit consumes 70% or more of the driving power used in the LCD device, and therefore affects the size, the light efficiency, mechanical and optical characteristics, etc. of the LCD. Also, the backlight unit can be classified into a direct type and an edge type based on the location of the light source that supplies light.
In more detail, when the brightness of the screen is not higher than 300 nits and the LCD must have a thin-shape such as for monitors or notebooks, the edge type backlight unit is used. The direct backlight unit is used for devices such as televisions, etc. Further, the edge type backlight unit includes a light source for generating light, and a light guide portion which converts the light generated at the light source into a uniform planar light. The light guide portion includes a light guide plate guiding the light across the surface of the LCD. That is, the light guide plate makes the light generated at the light source uniformly progress to all areas of the LCD panel.
However, because the edge type backlight unit has one or two light sources located at a side surface or two opposed side surfaces of the light guide plate, a brightness difference is generated between a region of the light guide plate close to and another region of the light guide plate remote from the light source. Therefore, the light guide plate has a pattern which disperses and scatteredly reflects the light incident into the light guide plate and transfers the dispersed and reflected lights toward the LCD panel. That is, the related art light guide plate does not uniformly distribute the light to the LCD panel.
In more detail, FIG. 1 is a cross-sectional view of a related art LCD device 10. As shown, the related art LCD device 10 includes an LCD panel 18 for displaying images, a backlight unit for applying light to the LCD panel 18, and a top case 60 covering the side of the LCD panel 18. In addition, the LCD panel 18 includes an array substrate 14 and a color filter substrate 16, which are bonded together with a fixed distance therebetween.
The array substrate 14 includes a plurality of pixels formed in a matrix shape, and the color filter substrate 16 includes at least three color filters formed in a matrix shape (e.g., R, G and B). Further, the backlight unit is located at the rear of the LCD panel 18 and applies light to the LCD panel 18. As shown in FIG. 1, the backlight unit includes a lamp 40, a light guide plate 30 disposed at one side of the lamp 40, and optical members 20 disposed over the light guide plate 30. The optical members 20 in FIG. 1 include a diffusion plate 21, a prism sheet 22 and a protection sheet 23.
In addition, the backlight unit is placed in and fixed to a bottom case 50. The bottom and top cases 50 and 60 are combined together to form a single unit. Further, the light guide plate is mainly fabricated using an injection molding process. However, the related art injection molding process for the light guide plate 30 has a problem in that residual stresses, such as a stress caused by a resin-filling unbalance and the stress generated by areas of the plate 30 cooling at different rates exist within the molded light guide plate 30.
Further, these residual stresses cause the light guide plate 30 to be deformed over time because of the high temperature they operate in. The deformed light guide plate 30 then presses neighboring elements contained within the LCD. For example, as shown in FIG. 1, the deformed light guide plate 30 presses the LCD panel 18 at the position “B”, which changes the liquid crystal alignment angle within the LCD panel 18. The changed liquid crystal alignment causes a picture quality defect such as a ripple phenomenon. The deformed light guide plate 30 also presses the bottom case 50 at the position “A” in FIG. 1, which negatively affects the optical pattern of the light guide plate 30, thereby causing a white spot defect.
For example, FIG. 2 is a photograph showing an example of a defect that appears on the screen of the LCD and that is caused by the light guide plate 30 bending. As shown in FIG. 2, a protruded or bent portion of the light guide plate 30 presses the optical members 20 and the LCD panel 18, thereby causing a ripple phenomenon effect on a screen 15 of the LCD 10. The lifetime of the light guide plate is also reduced.